What does SAE 15w40 and other letters on the oil bottles stand for?

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SAE 15W-40 refers to a multigrade lubricant. On this type of lubricant you’ll see two numbers separated by a hyphen. One of the numbers is followed by a W, which refers to “winter” and indicates the viscosity grade in cold temperatures. An oil with a low SAE W number will flow better, have better cold starts, and lower engine wear. The second number refers to the viscosity in warm temperatures (measured at 100ºC). The higher the SAE grade, the higher the viscosity and denser the lubricating film. Viscosity is necessary in warm temperatures to protect the mechanical pieces in contact. However, excessive viscosity at high temperatures could cause more internal friction and poorer engine performance and consumption.

Oil viscosity is very important and is affected by the temperature. SAE, the acronym for the Society of Automotive Engineers, is responsible for establishing a classification system based solely on oil viscosity. To classify the oils, their viscosity is measured in cold conditions at different temperatures below zero, and then in hot conditions at 100ºC. Using this base measurement, the oils are split into monograde (suitable for specific temperatures) or multigrade (suitable for a wide range of temperatures).

Multigrade oils, such as 15W-40, don’t need changing according to the seasons, just whenever it’s time for an oil change, as they are not affected by temperature in the same way as monograde oils.

Monograde oils have to be changed during seasons when weather conditions are at their most extreme (winter and summer). In summer you need more viscous oil because the heat makes it more liquid. In contrast, in winter you need a less viscous oil to make cold starts easier. If there is a letter W, it relates to an oil for winter. If instead of a W you only see numbers between 20 and 60, this refers to the oil’s viscosity at hotter temperatures.

As a general rule, you have to to follow the instructions given by the engine manufacturer which determine the most appropriate oil viscosity grade for the engine. The choice depends on the engine design, weather conditions, and what the vehicle is used for.